Three-dimensional nanostructures are widely utilized in a variety of fields including not only semiconductor-based microelectromechanical systems (MEMS), but also photonic crystal devices, display devices, etc.
Focused ion beam etching (FIBE) which is a kind of method for fabricating three-dimensional nanostructures enables ions formed in plasma to be accelerated, separately extracted and then focused so as to etch a specific portion of the substrate.
Because this method allows for the independent control of etching parameters, including the ion beam direction, ion flux, ion energy, etc., it has been extensively used in three-dimensional nanostructure fabrication. Also, a specimen may be etched using the physical energy of ions, advantageously forming three-dimensional nanostructures regardless of the type of specimen.
Copper, having high electrical conductivity, is being utilized in a variety of structures depending on the application purposes. Recently, thorough research into manufacturing micro-devices using three-dimensional copper nanostructures is ongoing. Three-dimensional copper nanostructures may be employed in cathodes for supercapacitors, secondary batteries or gas sensors, and may be applied in diverse fields, including MEMS (MicroElectroMechanical Systems), photonic crystal devices, etc.
To fabricate a three-dimensional copper nanostructure, controlling the angle and the aspect ratio of the nanostructure is essential. Ion beam etching for patterning a specimen using the physical energy of particles has been employed to date. The ion beam etching method is problematic because of ion implantation due to high bombardment energy of ions, lattice defects of a specimen, etching shape distortion due to re-deposition of sputtered particles, etc. Furthermore, this method makes it impossible to manufacture a large-area structure and is thus unsuitable for use in commercialization.
The techniques related with nanostructure fabrication are disclosed in Korean Patent Nos. 0281241 and 1249981, which are referred to as “Conventional Technique 1” and “Conventional Technique 2,” respectively.
Below is a description of a plasma etching method using a Faraday cage with the upper grid plane of various geometries, and a three-dimensional nanostructure and a fabrication method thereof, as disclosed in Korean Patent Nos. 0281241 and 1249981.
FIG. 1 is a cross-sectional view illustrating an etching device wherein a Faraday cage having a grid plane slanted relative to a substrate is brought into electrical contact with the cathode of a TCP plasma etching reactor, according to Conventional Technique 1.
With reference to FIG. 1, the etching method using a Faraday cage 14 enables a substrate to be comparatively simply patterned under high-density plasma conditions, giving a slanted etch profile. The Faraday cage 14 refers to a closed space made of a conductor. When the Faraday cage 14 is provided in plasma, a sheath is formed on the outer surface of the cage, and an electric field is maintained constant therein. As such, when the top of the cage is replaced with a fine grid, the sheath is formed along the surface of the grid. Thus, the ions, which are accelerated in the sheath formed horizontally to the surface of the grid, are incident into the cage and then arrive at the substrate while the incident directionality thereof is maintained. Thus, when the specimen is disposed at different specimen holder gradients, the ion incident angle may be arbitrarily adjusted. The use of such a Faraday cage 14 may advantageously result in one-step fabrication of a slanted etching structure.
However, the etching method using the Faraday cage 14 according to Conventional Technique 1 is disadvantageous because it may be applied when only a material which may be etched with plasma is used as a target.
For example, as illustrated in FIG. 2, in the case where copper is etched using plasma, a volatile copper compound cannot be formed, and thus etching does not take place. Thus, methods of fabricating three-dimensional copper nanostructures using plasma have not yet been introduced, and no solutions have been found to date.
FIG. 3 illustrates a process of fabricating a three-dimensional nanostructure according to Conventional Technique 2. As illustrated in FIG. 3, the method of fabricating the three-dimensional nanostructure according to Conventional Technique 2 includes (a) sequentially forming a target layer 120 and a polymer layer 130 on a substrate 110; (b) performing lithography on the polymer layer 130 to form a patterned polymer structure 135; (c) subjecting the target layer 120 to ion etching, thus forming a target-polymer composite 150 in which the ion etched target is attached to the outer surface of the polymer structure 135; and (d) removing the polymer from the target-polymer composite 150, thus manufacturing a three-dimensional nanostructure 200.
However, the three-dimensional nanostructure according to Conventional Technique 2 suffers from, in the course of forming the nanostructure using ion etching, ion implantation due to high bombardment energy of ions, lattice defects of a specimen, etching shape distortion due to re-deposition of sputtered particles, etc. Additionally, this method makes it impossible to manufacture a large-area structure and is thus inappropriate for commercialization.